1. Field of the Invention
The present invention relates to a light emitting device used in a backlight of a portable telephone, a display capable of showing various data, a light source for various sensors such as a line sensor, an indicator, and to a light emitting device used for an indication of various measuring instruments and outdoor guide plates. The present invention especially relates to a light emitting device having a high reliability and one that exhibits excellent heat resistance and light stability.
2. Discussion of the Related Art
In a conventional light emitting device, a light emitting element is mounted on a predetermined substrate and an encapsulating member is disposed to cover the light emitting device. The light emitting element is bonded to a desired member such as a lead frame or a printed wiring substrate, and molded with an encapsulating member which is made of a light-transmissive resin. An organic material, such as epoxy resin and silicone resin, is usually used for the encapsulating member. With this light emitting device, light is emitted from the light emitting element by applying electric current to the light emitting element. The emitted light penetrates the encapsulating member such as an epoxy resin and is transmitted through the air.
When an encapsulating member of an organic material, such as epoxy resin and silicone resin, is used to cover the light emitting element, there have been cases where the organic material deteriorates due to the heat and light generated from the light emitting element. There have also been cases where a component in an organic material causes deterioration of a printed wiring substrate, an electrode, and the like. As a result, a decrease in the optical characteristics, the electric characteristics, and the reliability characteristics of the light emitting device may occur.
A chip-type light emitting device has been known, in which an organic material such as epoxy resin is replaced with a low-melting point glass in order to solve such a problem. (For instance, see Japanese Unexamined Patent Publication Nos. 11-177129, 2002-203989, and 2004-200531.) FIG. 23 is a schematic cross-sectional view showing a conventional chip-type light emitting device 2000. In the chip-type light emitting device 2000, a light emitting element 2010 bonded to a printed wiring substrate 2020 is covered with an encapsulating member 2030 of a low-melting point glass. Lead glass is used for the low-melting point glass. The melting point of lead glass is about 400° C. Melting or hardening lead glass at a temperature of about 400° C. does not affect the properties of the light emitting element 2010. The chip-type light emitting device 2000 is manufactured by bonding the light emitting element 2010 to the predetermined printed wiring substrate 2020. An electrode of the light emitting element 2010 and a wiring base 2021 are wire-bonded using a wire 2090. The printed wiring substrate 2020 having the light emitting element 2010 bonded thereto is covered with a predetermined metal mold. A hole for injecting an encapsulating member is made in the metal mold. A liquefied low-melting point glass flows into the metal mold from the hole and is then hardened to bond to a material substrate. After bonding, the metal mold is removed, and by cutting to a predetermined size, the chip-type light emitting device 2000 is obtained.
However, when manufacturing the conventional light emitting device 2000, the low-melting point glass is melted into a liquid, so that when the low-melting point glass is hardened, disconnection of wire 2090 or detachment of the light emitting element 2010 from the printed wiring substrate 2020 may occur. This is due to the differences in the expansion coefficients of the printed wiring substrate 2020 and the low-melting point glass that is the encapsulating member 2030 when hardening the low-melting point glass. Moreover, the low-melting point glass has a low light extraction efficiency. This is because the low-melting point glass has a color and a portion of the emitted light from the light emitting element 2010 is absorbed by the colored portion of the low-melting point glass. Further, the low-melting point glass is susceptible to heat and humidity and its chemical stability is low.